1. Field of the Invention
The present invention relates to a display device and a method for fabricating the display device.
2. Description of the Related Art
Recently, flat-panel displays (FPDs) such as liquid crystal displays (LCDs) have been used extensively. Also, so-called “electronic paper” display devices having a further reduced thickness have been researched and developed.
A liquid crystal display device conducts a display operation by utilizing variations in the electro-optical properties of its liquid crystal layer. Such variations are caused by liquid crystal molecules that change their orientation directions in response to an electric field applied thereto. The electric field is normally applied to the liquid crystal layer by creating a voltage between a pair of electrodes that faces each other with the liquid crystal layer interposed between them. The structure of that pair of electrodes for use to apply the electric field to the liquid crystal layer is changeable with the specific mode of operation of the liquid crystal display device.
A typical electrode structure will be described with respect to a reflective active-matrix-addressed liquid crystal display device as an example. In an active-matrix-addressed liquid crystal display device, the pair of electrodes for use to apply an electric field to its liquid crystal layer normally includes a pixel electrode, which is provided on an active-matrix substrate, and a counter electrode, which is provided so as to face the pixel electrode.
A reflective liquid crystal display device includes a reflective layer to conduct a display operation by getting an incoming light ray modulated by the liquid crystal layer and then reflected by the reflective layer. In some reflective liquid crystal display devices, the pixel electrodes thereof also function as the reflective layer. The pixel electrodes with such a reflection function (which are sometimes called “reflective pixel electrodes”) may be obtained by using a metal having a high reflectivity as a material for the pixel electrodes. Those reflective pixel electrodes may have their surface patterned into any of various shapes so as to have either scattering (or diffusing) reflective properties (see Japanese Patent No. 3187369, for example) or retro-reflective properties (see Japanese Patent Application No. 2000-096075, for example).
In a transmissive liquid crystal display device on the other hand, the pixel electrodes thereof are normally transparent electrodes and often define a flat layer with a surface that is parallel to that of the liquid crystal layer. However, to improve the viewing angle characteristics of the liquid crystal display device by orienting the liquid crystal molecules in various directions, those transparent pixel electrodes may also have their surface shapes controlled in such a manner as to utilize either shape effects or inclined electric field effects.
In such a liquid crystal display device, the pixel electrodes with that controlled surface shape are provided over an active-matrix substrate. More specifically, an interlevel dielectric layer is formed on the active-matrix substrate so as to have a predetermined surface shape and then the pixel electrodes are formed on the interlevel dielectric layer. That is, the surface shape of the pixel electrodes is controlled by the surface shape of the interlevel dielectric layer on which the pixel electrodes are formed. Also, those pixel electrodes are electrically connected to active components by way of contact holes that are provided through the interlevel dielectric layer.
The conventional liquid crystal display devices, however, have the following drawbacks. Hereinafter, the problems of an active-matrix-addressed liquid crystal display device to be fabricated by a conventional method so as to have retro-reflective properties will be described with reference to FIGS. 11A through 11D.
First, as shown in FIG. 11A, a gate electrode 1102, a source electrode 1103, a drain electrode 1104 and a connector electrode 1105 are formed on a glass substrate 1101. In FIG. 11A, a semiconductor layer (including a channel region) which covers the gate electrode 1102 is not shown for the sake of simplicity. The substrate including these members thereon will be simply referred to herein as a “substrate 1107”. The gate, source, drain electrodes 1102, 1103, 1104 and the semiconductor layer (including a channel region) together make up a TFT. Although not shown in FIGS. 11A through 11D, a gate-bus line and a source-bus line are respectively connected to the gate electrode 1102 and the source electrode 1103 of the TFT. The connector electrode 1105 is electrically connected to the drain electrode 1104 and will be connected to a pixel electrode 1110 through a contact hole 1109 to be described later. Optionally, the connector electrode 1105 may be omitted. In that case, the contact hole 1109 needs to be provided over the drain electrode 1104 such that the drain electrode 1104 will be directly connected to the pixel electrode 1110.
Next, as shown in FIG. 11B, an undercoat film 1108 for a retro-reflective layer is bonded onto the substrate 1107 by the method disclosed in U.S. Pat. No. 4,601,861, for example. The undercoat film 1108 is made of an insulator (typically a resin).
Subsequently, as shown in FIG. 11C, a contact hole 1109 is formed through the undercoat film 1108 by a photolithographic process, for example, so as to be located over the connector electrode 1105. That is to say, a portion of the connector electrode 1105 is exposed inside the contact hole 1109 that has been formed through the undercoat film 1108.
Thereafter, as shown in FIG. 11D, a pixel electrode 1110 is formed on the undercoat film 1108 so as to be connected to the connector electrode 1105 by way of the contact hole 1109. The pixel electrode 1110 may be formed by selectively depositing a metal material (e.g., Al) on the substrate by an evaporation process with unnecessary regions masked. Alternatively, a film of the metal material may be deposited over the entire surface of the substrate 1107 and then patterned into the predetermined shape by a photolithographic process, for example. The pixel electrode 1110 is a retro-reflective pixel electrode that also functions as a retro-reflective layer. In this manner, an active-matrix substrate is obtained.
Finally, the active-matrix substrate including the retro-reflective pixel electrode 1110 is bonded with a counter substrate (not shown), which has been prepared separately, with a predetermined gap provided between them. The counter substrate includes a color filter (CF) layer and a counter electrode, which are stacked in this order on a glass substrate, for example. The color filter layer includes red (R), green (G) and blue (B) color filters and optionally includes a black matrix. The counter electrode may be made of indium tin oxide (ITO), for example. Then, a scattering type liquid crystal material (e.g., a polymer dispersed liquid crystal material) is injected into the gap between the active-matrix and counter substrates, thereby obtaining a retro-reflective liquid crystal display device.
To achieve an ideal retro-reflection property, the surface shape of the retro-reflective layer needs to consist of two groups of planes that are tilted in mutually different directions with respect to the surface of the glass substrate 1101 (i.e., a plane that is parallel to the display screen) as schematically shown in FIGS. 12A and 12B. Also, these two groups of planes need to define a regular repetitive pattern. However, the pixel electrode 1110 formed by the conventional method includes a flat portion over the contact hole 1109 as shown in FIG. 11D. Thus, the cross-sectional shape of the retro-reflective pixel electrode 1110 is different from the rugged surface shape of the ideal retro-reflective layer.
Furthermore, in the regular rugged structure of the ideal retro-reflective layer, the depth L of the rugged structure (i.e., difference in vertical level between the highest-level points 1101 and the lowest-level points 1102) is obtained by multiplying the pitch P by √{square root over (6)} and dividing the product by 3 (i.e., L=SQRT (6)*P/3). Accordingly, to achieve the ideal retro-reflection property, the thickness of the undercoat film 1108 needs to be greater than the depth L of the rugged structure. That is to say, the depth of the contact hole 1109, which is used to electrically connect the connector electrode 1105 under the undercoat film 1108 to the retro-reflective pixel electrode 1110 on the undercoat film 1108, needs to be greater than the depth L of the rugged structure.
If the contact hole 1109 has a depth of about 1 μm or more, then it is normally difficult to cover the entire inner surfaces of the contact hole 1109 with the metal material of the pixel electrode 1110 by a thin-film deposition process. For that reason, to achieve good electrical connection by filling the contact hole 1109 with the metal material, the inner surfaces of the contact hole 1109 may be tapered. In that case, however, the diagonal size of the contact hole 1109 (i.e., its area when the contact hole 1109 is projected onto the surface of the substrate 1101) will increase. As a result, the portion of the pixel electrode 1110 over the contact hole 1109, which has a different surface shape from the other portions thereof, will increase its area.
Hereinafter, the problems of the retro-reflective pixel electrode 1110 made by the conventional method will be described in further detail with reference to FIGS. 13A and 13B.
The retro-reflective pixel electrode 1110a shown in FIG. 13A is connected to the connector electrode 1105 inside the contact hole 1109 that runs through the interlevel dielectric layer 1108a. Accordingly, the surface shape of that portion of the retro-reflective pixel electrode 1110a, which is located inside and around the contact hole 1109 (and will be referred to herein as a “contact hole portion”), is greatly different from the predetermined shape 1110aR of the retro-reflective pixel electrode 1110a. A portion of the pixel electrode that is electrically connected to the connector electrode (or drain electrode) will be referred to herein as a “contact portion”. If the contact portion is located inside a contact hole, a portion of the pixel electrode, which covers the contact hole and has a different surface shape from that of the other portions thereof, will be referred to herein as a “contact hole portion”.
The distribution of the tilt angles Φ that are defined by the surface of the retro-reflective pixel electrode 1110a with the display screen (i.e., the surface of the glass substrate) is schematically shown in the lower portion of FIG. 13A. As shown in FIG. 13A, the tilt angle Φ steeply changes near the contact hole 1109 and significantly deviates from the ideal tilt angle ΦR of the retro-reflective pixel electrode 1110a. Also, a flat portion with a tilt angle Φ of zero degrees is present at the center of the contact hole 1109.
As described above, if the pixel electrode 1110a is electrically connected to the connector electrode 1105 inside the contact hole 1109, then the surface shape of the pixel electrode 1110a will be greatly different from the predetermined shape there. Thus, the intended retro-reflection property is not achievable. As a result, the effective display area virtually decreases, the contrast ratio drops, and the display quality degrades eventually.
On the other hand, as shown in FIG. 13B, if a contact hole 1109, smaller than the contact hole 1109 shown in FIG. 13A, is provided through a portion of the interlevel dielectric layer 1108b that includes a lowest-level point 1102 of the retro-reflective pixel electrode 1110b (see FIG. 12A), then the flat portion with the tilt angle Φ of zero degrees will have a decreased area. However, the tilt angle changes even more steeply near the contact hole 1109 and the intended retro-reflection property is not achievable, either.
Such a phenomenon occurs in not just reflective display devices including a retro-reflective layer but also reflective display devices including a scattering reflective layer.
For example, in the reflective display device disclosed in Japanese Patent No. 3187369, the rugged surface shape of a scattering reflective layer is defined by the tilt angles that are formed by the surface with respect to the display screen. However, even if the surface shape of the scattering reflective layer is optimized, the scattering reflective layer also has a surface shape greatly different from the predetermined one in that contact hole portion as long as the reflective display device is manufactured by the conventional method. Thus, the intended scattering reflection property is not achievable, either.
As schematically shown in FIG. 14A, the portion of a scattering reflective electrode 1110c, which is located inside the contact hole 1109, has an almost entirely flat (i.e., Φ=0) surface shape. Accordingly, the tilt angle Φ also changes steeply, and the surface shape is also greatly different from the predetermined surface shape 1110cR representing the ideal scattering reflection property, near the contact hole 1109. As a result, the intended scattering reflection property is not achievable, either.
A similar problem may also occur in transmissive liquid crystal display devices, not just the reflective liquid crystal display devices described above. For example, to increase the aperture ratio of a transmissive liquid crystal display device, a transparent pixel electrode 1110d may be provided on a transparent interlevel dielectric layer 1108d as shown in FIG. 14B. In such a configuration, if the inner surfaces of the contact hole 1109 are tapered (where Φ≦45 degrees) to electrically connect the pixel electrode 1110d to the connector electrode 1105 just as intended, then the tilt angle Φ also changes near the contact hole 1109. In that case, the orientation directions of liquid crystal molecules also change near the contact hole 1109, thus possibly degrading the display quality.
Furthermore, even in a transmissive liquid crystal display device, the surface of the pixel electrode may also have to be roughened to control the orientation directions of the liquid crystal molecules. In that case, if the surface shape of the pixel electrode is not the predetermined one near the contact hole, the display quality might also degrade.
The problems described above may arise not only in the active-matrix-addressed liquid crystal display devices but also in simple-matrix-addressed liquid crystal display devices as well. Furthermore, similar problems may also happen in any other display device with a display medium layer exhibiting electro-optical effects, not just those liquid crystal display devices.